Active array antennas for use in radar systems may include many hundreds or thousands of antenna elements to achieve the desired beamwidth and number of beams. Each of the antenna elements, or each set of the antenna elements of an active array, is associated with an active module. Each active module operates on the radio-frequency (RF) signal transduced by the associated element(s), as by amplification, phase shifting, amplitude control, or the like. In this context, "active" means a module which requires, for its operation, electrical bias separate from the RF signals on which it operates. Passive RF operations are possible, such as phase shift, but in the absence of active elements, they can only be changed by mechanical means, as by adjusting a capacitor. Since one of the major advantages of an array antenna is that it may be "agile" (the beam can be quickly slewed and changed) when electronically controlled, almost all array antennas include active modules associated with the antenna elements. Such modules may be transmit-only, receive-only, or they may be transmit-receive (T/R or TR) modules when the array antenna is used for both transmission and reception. U.S. Pat. No. 5,339,086, issued Aug. 6, 1994, in the name of DeLuca et al., describes in general how TR modules are controlled in an array antenna context. U.S. Pat. No. 5,280,297, issued Jan. 18, 1994 in the name of Profera, describes the use of active modules for retransmission in the context of a reflectarray.
In a radar context, the transmissions, as for example transmit pulses, must have relatively high power. The power is achieved by "summing in space" the powers radiated by each of the antenna elements. Since the radiated RF power originates within the TR modules, the total energization power applied to the TR modules must be at least the sum of the transmitted RF power plus the losses in the TR module. In turn, the power requirements of a powerful radar system may be considerable, with the principal portion of the power going to the TR modules. Thus, operation of a radar system using an active array antenna requires a power source for the TR modules. U.S. Pat. No. 5,173,706, issued Dec. 22, 1992 in the name of Urkowitz, describes an active array antenna in the context of a radar system.
Ideally, the active portions of at least the analog components of a radar system are energized by pure direct voltage, also known as direct current (DC), to avoid the effects of noise on the direct voltage buses. For generating low-noise direct energizing voltages for the radar system, elaborate power-line filters and regulators are the norm. Those skilled in the art know that, notwithstanding these precautions, the direct energizing voltage always contains some residual noise, and that the circuits which are driven thereby should ideally be designed to reject energization voltage noise. Even when so designed, some residual modulation of the RF by the noise can occur, generally at the power-line frequency of 60 Hertz (Hz.) or its harmonics for ground-based systems, and 400 Hz. or its harmonics for airborne systems. The residual modulation, in turn, tends to adversely affect some of the normal functions of the radar, and especially those which depend for their operation upon cancellation of like signals, in which minuscule differences between the signals being canceled may result in an undesired residual signal. One function which is adversely affected by energization voltage noise is the clutter improvement factor (CIF), in which repetitive signals (clutter) are phase-shifted in alternate time periods, and added together in such a manner that they cancel.
Improved array antenna systems are desired.